When document data created on application software in a host computer is to be printed by an image forming apparatus such as a color printer, the document data is generally processed by the following procedures.
The document data is converted into a print job described in the PDL (Page Description Language) by a printer driver in the host computer. The print job is then transmitted to the color printer.
Upon reception of the print job, the color printer executes RIP (Raster Image Processing) for the print job by a controller to rasterize the print job into a bitmap image. The rasterized bitmap image is then sent to a print engine.
In general, there are various types of print engines used in color printers. For example, an electrophotographic print engine irradiates a photosensitive drum with light corresponding to an image signal from an exposure means such as a laser, and electrostatically develops with toner a latent image formed on the photosensitive drum. The print engine electrostatically transfers the toner image onto a sheet, and fuses and thermally fixes the toner image by a fixing means, forming the image on the sheet. Printing ends with the above processing.
The print engine changes the tint of an output image to be printed upon variations in use environment such as the air temperature or humidity, changes in the apparatus over time, deterioration of the performance of a member upon endurance, and the like. In order to obtain a stable output while suppressing variations in tint caused by these factors, various calibration techniques have been proposed for color printers.
The calibration technique includes many methods depending on the difference in the type of print engine, the difference in measurement means, and the like. Basically, a measurement sample which reflects the output characteristic of the print engine is printed by the print engine. The measurement sample is measured by any measurement means, and a correction table is created on the basis of the measurement result and target data having preset ideal reproducibility.
A print job transmitted from the host computer is corrected on the basis of the correction table in RIP or subsequent processing, and the contents of the correction table are reflected in final printing from the print engine.
In addition, the controller performs various image processes in RIP. In general, several choices are prepared for settings of image processing and can be selected by the user in order to meet various printing purposes of the user. These settings can be made as print setting options via the operation unit of the printer driver or controller.
An example of items whose settings can be changed as print setting options is switching of the halftone screen. As a characteristic of the halftone, a high-LPI halftone is suitable for a text and a graphic image mainly formed from a line art, whereas a low-LPI halftone is suitable for an image such as a photographic image. The user can select the halftone in accordance with the pattern of an image to be printed. The output characteristic changes between a high LPI and a low LPI.
An image to be printed may change depending on selected items associated with the print engine out of items selected by the user as print setting options, in addition to settings of image processing. For example, various sheets are used for printing, and even paper varies in thickness and texture. An image to be printed changes greatly depending on what kind of print medium is used.
Some electrophotographic color printers are equipped with a glossiness control function for controlling the fusibility of toner on a sheet by changing the fixing speed in fusion and thermal fixing. If the glossiness is controlled in accordance with an instruction from the user via the printer driver, not only the glossiness but also the density and color reproducibility generally change.
As described above, the color printer may change the density and color of an image to be printed depending on the setting contents of the above-mentioned setting items out of print setting options. However, a purpose of these setting items is not to intentionally adjust the tint, unlike a color tone correction function, monitor matching function, and ink color simulation function. For this reason, it is ideal to obtain color reproducibility which always exhibits a constant density and color regardless of setting contents.
A conventional calibration technique can generate a correction LUT (Look Up Table) for correcting a tint difference caused by a setting difference in print setting options except options for intentionally adjusting the tint.
For example, there is a technique of correcting a tint difference caused by a difference in halftone screen. According to this technique, all halftone screens installed in the print engine are sequentially designated to print measurement samples. The density of each color patch is measured to create a correction LUT for each halftone screen. Further, by designating a sheet tray in printing a measurement sample for correcting a halftone screen, a correction LUT obtained by measuring a measurement sample can be prepared for each combination of a sheet tray and halftone screen (see U.S. Pat. No. 5,936,741).
However, according to this conventional technique, correction LUTs for correcting a tint difference caused by a setting difference in print setting options except options for intentionally adjusting the tint must be prepared by the number of all combinations of choices of print setting options to be set. If the number of print setting options to be set is increased, the number of combinations greatly increases. It becomes cumbersome to perform calibration operation for all necessary combinations.
In order to obtain an appropriate calibration effect in print operation, the user must select a proper one of many correction LUTs. It becomes very difficult to select a proper correction LUT when the number of correction LUTs is extremely large. For this reason, it is substantially impossible to correct the tint for print setting options except options for intentionally adjusting many tints.
In print operation, the user must select a proper correction LUT in accordance with a combination of selected print setting options. If, however, the number of correction LUTs is large, an improper correction LUT may be designated and applied, undesirably varying the tint.